The present invention relates to an amplitude limiter operating at audio frequencies, and it is particularly intended for a deviation limiter in a mobile telephone or a base station.
In the transmitter of a radio telephone, the amplitude of the input voltage of the FM modulator has to be limited in order to limit the frequency deviation of an FM-modulated signal. Signals of an excessively high level are clipped by a level clipper which limits the amplitudes of the signals to a level between a higher level and a lower level. A normal way of implementing the level clipper is to form limiting levels on the voltage divider directly from the supply voltage or to use a reference voltage formed in relation to the supply voltage.
This known method has the disadvantage that variations and disturbances in the supply voltage affect the reference voltages and thereby also directly the limiter output signal. In order to eliminate these disadvantages the disturbance components within a wide frequency range must be filtered out from the supply voltage, and efforts must be made to keep the absolute value of the supply voltage as stable as possible. This for its part will lead to the use of expensive precision components. Such specialized components include precision capacitors, precision resistors and a regulator, which have been used in prior-art mobile telephones and base stations implemented using, for example, an NE 571 circuit. However, it has been difficult to achieve sufficiently precise limiting levels and sufficiently precise build-up and decay times.